1. Field of the Invention
The present invention relates to a nonvolatile memory device having memory cells and corresponding reference cells.
2. Description of the Related Art
Examples of nonvolatile memory devices using resistance material include resistive random access memory (RRAM) devices, phase change random access memory (PRAM) devices, magnetic random access memory (MRAM) devices, and the like. Dynamic random access memory (DRAM) devices or flash memory devices store data by charges, while the nonvolatile memory devices using resistance material store data by changing the resistance of a variable resistance material (corresponding to RRAM), changing the state of a phase change material, such as a chalcogenide alloy (corresponding to PRAM), changing the resistance of a magnetic tunnel junction (MTJ) thin film according to a magnetization state of a ferromagnetic material (corresponding to MRAM), and the like.
Resistive memory cells include variable resistance material provided between an upper electrode and a lower electrode, and the resistance level of the variable resistance material changes according to a voltage applied to the upper and lower electrodes. In particular, a filament that serves as a current path of a cell current is formed in the variable resistance material. A state in which a part of the filament is disconnected is defined as a reset state, a high resistance state, and reset data (data 1). A state in which the filament is connected is defined as a set state, a low resistance state, and set data (data 0).
A reset voltage, which has a voltage level at which the filament may be disconnected, is supplied to write reset data into the resistive memory cells. A set voltage, which has a voltage level at which the filament may be reconnected, is supplied to write set data into the resistive memory cells. Further, a voltage that has a low voltage level at which the state of the filament does not change is supplied to read the stored data to determine whether the read data is reset data or set data.